Do you have the 3D Vision driver installed? Check “Control Panel -> Program and Features” and look for “nVidia 3D.”

Apparently there’s a bug with The Witcher 2 that absolutely tanks performance if you have the 3D vision drivers installed – it doesn’t matter whether or not you’re using them. I had this problem and in parts I was getting as low as 12 FPS on my GTX 570. Uninstalling the 3D Vision drivers fixed everything and in the same sections I now get 60+ fps with everything maxed except Ubersampling.

I was very lucky to stumble upon this information. Spread the word.

Of course also make sure you have the latest drivers from nVidia, which are 275.27 as of writing this. I heard those can solve a lot of problems as well.